A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to control the lithographic process to place device features accurately on the substrate, alignment marks are generally provided on the substrate, and the lithographic apparatus includes one or more alignment sensors by which positions of marks on a substrate can be measured accurately. These alignment sensors are effectively position measuring apparatuses. Different types of marks and different types of alignment sensors are known from different times and different manufacturers. A type of sensor widely used in current lithographic apparatus is based on a self-referencing interferometer as described in U.S. Pat. No. 6,961,116 (den Boef et al). Generally marks are measured separately to obtain X- and Y-positions. However, combined X- and Y-measurement can be performed using the techniques described in published patent application US 2009/195768 A (Bijnen et al). The contents of both of these applications are incorporated herein by reference.
A current alignment technique comprises illuminating the alignment mark and obtaining an interference pattern from the +1st and −1st diffractive orders, with the 0th order being blocked. This is sometimes referred to as dark field detection. However, the 1st order diffraction efficiency of alignment marks is tending to become smaller (as the alignment mark contrast is becoming lower), meaning that the 1st order signals are increasingly weaker. In addition to this, there is a growing need to measure each alignment mark in a shorter time. Shot-noise limited detection of such weak signals in a short time therefore becomes impossible without impractically powerful lasers. Another limitation of dark-field detection is its limited capability to detect asymmetric deformation of an alignment mark. Processing steps like etch and Chemical Mechanical Polishing (CMP) can deform the mark which leads to an alignment offset. Detection of this deformation is of value and can be done by measuring an intensity imbalance between the +1st and −1st order. In dark field detection, however, these orders are added which makes it impossible to detect the asymmetry.